Electron lens



E. T. RATE, JR

May 3, 1966 ELECTRON LENS Filed Nov. 9, 1962 FIG.|.

FIG.3.

PRIOR ART INVENTOR DWARD T.

RATE,JR.

HIS TTORNEY.

United States Patent 3,249,786 ELECTRON LENS Edward T. Rate, Jr., North ,Syracuse, N.Y., assignor to General Electric Company, a corporation of New York Filed Nov. 9, 1962,Ser. No. 236,624

3 Claims. (Cl. 31382) The present invention relates to improvements in cathode ray tubes, and more particularly to an improved electrostatic electron lens having minimal spherical aberration, particularly suitable for focusing electrons emitted from a source into an electron. beam of minimal crosssectional area.

One object of the present invention is to provide an improved electrostatic electron lens particularly suitable for condensing electron beams in high resolution cathode These and other objects of the invention will be ap-' parent from the following description and the accompanying drawing wherein:

FIGURE 1 is a fragmentary view, partially broken away in axial section, of an electron lens system constructed according to the present invention.

FIGURE 2 is a partially broken away view of a cathode ray tube having an electron lens such as shown in FIGURE 1; and

FIGURE 3 is a fragmentray view of prior art over which the present invention affords improved performance; and

FIGURE'4 is an enlarged view of a portion of the structure of FIGURE 1.

Referring to FIGURE 1 of the drawing, an electron lens constructed according to my invention includesa sheet metal cup-shaped electrode 2 having a cylindrical sidewall 4 arranged coaxially with the-reference axis 6 of a prospective electron beam path, and an integral end wall 8. The end wall 8 is reentrant within the cylindrical side wall 4 and forms an outwardly concave conical surface 10 coaxial with the reference axis 6 and having an apex angle of 109". At the apex of conical surface 10 is provided a small round aperture 12 coaxial with reference axis 6. If desired for ease in producing the aperture 12, the portion of end wall 8 including the aperture 12 and extending radially a few mils beyond the periphery of the aperture 12 may be pressed, drawn or otherwise formed flat, as best shown in FIGURE 4, in a plane transverse to the axis of the conical surface, prior to producing the aperture. After the aperture 12 is formed, a small annular flange 14 transverse to the axis of conical surface 10 may remain, integrally connected to the conical surface 10 by a short substantially cylindrical throat 16 drawn during the formation of flat surface 14. The outer diameter of conical surface 10 should preferably be at least ten to twenty times that of aperture 12. The peripheral portion of conical surface is smoothly curved around into sidewall 4, as at 22, so as to minimize abrupt space potential gradients adjacent to the juncture of these two surfaces.

Coaxially disposed in a transverse plane directly opposite conical surface is a cylindrical accelerator electrode 30. The end of accelerator electrode facing the conical potential gradients, and has a diameter approximately equal to the outer diameter of conical surface accelerator electrode may have one or more coaxially apertured transverse internal partitions 34 if desired.

As best shown in FIGURE 1, a cathode 40 having an electron emitting surface 42 is disposed coaxially behind aperture 12 so as to provide a source of an electron beam for passage through aperture 12. Intensity control of the electron beam from surface 42 is provided by a cup shaped control grid 44 having a surface 46 transverse to axis 6 spaced between cathode 40 and aperture 12 and having a coaxial opening 48 for passage of the electron beam.

As best shown in FIGURE 2, the electrode system of FIGURE 1 is particularly suited for forming in a cathode ray tube, a high resolution beam of electrons which after passing through electrode 30, is focused by an external focus coil 60, deflected by external deflection coils 62, and thereby caused to scan a luminescent material target 64 with an extremely small electron beam landing spot size at the target. Suitable exemplary potentials for the electrodes of the lens system in the tube of FIGURE 2 are zero for the cathode, --50 to 150 volts for the control grid, 1500 volts for the conical electrode 2, and 20,000 volts for the accelerator electrode 30.

In operation of a tube such as shown in FIGURE 2 it has been found that a substantial reduction in spot size for a given beam current to the target, or conversely a substantial increase in beam current to the target for a given spot size, results from substitution of my improved electron lens for the so-called dip-in type of lens employing telescoped cylindrical cups 70 and 72 with flat transverse bottom walls 74 and 76 respectively as shown in FIGURE 3. For example, in a tube such as shown in FIGURE 2, I have been able to readily obtain spot sizes of .0005 inch diameter (as measured by the shrinking raster method) with anode currents of one microampere, or spot sizes of .0007 inch with anode currents of 5 microamperes, which performance compares very favorably with that of prior art apparatus such as shown in FIGURE 3 where minimum spot size at 1 microampere was .0007 inch. This improved performance is believed to result from, and be representative of, the minimal spherical aberration of the electron beam condensing electro- 7 The surfaces 80 are believed to be a family of hyperboloids coaxial with axis 6 and coasyrnptoti-c with conical surface 10. Desirably electrode 30 should be axially spaced from surface 10 a minimum distance consistent with voltage breakdown considerations, to shield the region of equipotentials 80 from external disturbing influences. By way of illustration, for example, I have had excellent results with electrodes 30 and conical surface 10 having outer diameters of about one half inch, respective potentials of 1500 and 20,000 volts, and an axial spacing as shown in FIGURE 1 of about one quarter inch.

It will be appreciated by those skilled in the art that the invention may be carried out in various ways and may take various forms and embodiments other than the illustrative embodiments heretofore described. Accordingly it is to be understood that the scope of the invention is not limited'by the details of the foregoing description,-

1. In a cathode ray tube having a reference electron beam axis, a first electrode having a concave conical surface of about 109 apex angle coaxial with said reference axis, said first electrode having an aperture coaxial with said reference axis at the apex of said conical surface and through which an electron beam can be admitted into the region surrounded by said conical surface, the peripheral portion of said conical surface at the maximum diameter thereof being outwardly and rearwardly curved back upon itself to form a rounded peripheral edge on said conical surface, and an accelerating electrode having a cylindrical portion coaxially spaced from said conical surface and having an open end disposed in coaxial spaced confrontingrelation with said conical surface, the peripheral portion of said open end being outwardly and rearwardly curved back upon itself to form a rounded peripheral edge on said cylindrical portion, said open end having a diameter approximately equal that of the maximum diameter of said conical surface, the axial distance between the peripheral edge on said conical surface and the peripheral edge on said cylindrical portion being about /2 the maximum diameter of said conical surface, the maximum diameter of said conical surface being at least ten times that of said aperture.

2. In a cathode ray tube, a cathode having an emitting surface arranged to emit electrons for formation into an electron beam having a reference axis, an apertured transverse control electrode next adjacent to and spaced coaxially from said cathode along said reference axis, a first electron accelerating electrode spaced from and next adjacent said control electrode along said reference axis and having a coaxial conical surface concave away from I said cathode and apertured at its apex for passage of the electron beam, the apex angle of said conical surface being about 109, the peripheral portion of said conical surface at the maximum diameter thereof being outwardly and rearwardly curved back upon itself to form a rounded peripheral edge on said conical surface with an axially extending cylindrical skirt portion and a second accelerating electrode having a cylindrical portion coaxially disposed opposite said conical surface and provided with an open end confronting said conical surface, the peripheral portion of said confronting end being outwardly and rearwardly curved back upon itself to form a rounded peripheral edge on said confronting end, the opening of said confronting open end having a diameter approximately equal that of the maximum diameter of said conical surface, the distance between the said peripheral edge of said conical surface and the said peripheral edge of said cylindricalportion being about /2 the maximum diameter of said conical surface, the maximum diameter of 1 said conical surface being at least ten times that of said apex aperture.

3. In a high resolution formation into an electron beam having a reference axis, an apertured control electrode next adjacent to and spaced from said cathode along the reference axis,'a first electron accelerating electrode spaced from said control electrode along said reference axis and having a coaxial conical surface concave away from said cathode, a hollow throat section projecting axially outwardly from the apex of said conical surface for passage of, the electron beam, an apertured transverse radially inwardly directed flange on the end of said throat section defining an aperture therethrough, the apex angle of said conical surface being about 109 and the maximum diameter of said conical surface being at least ten times that'of said aperture, the peripheral portion of said conical surface at the maximum diameter thereof being outwardly and rearwardly curved back upon itself to form a rounded peripheral edge on said conical surface and an axially extending cylindrical skirt, and a cylindrical accelerating electrode having a cylindrical portion coaxially disposed opposite said conical surface and next adjacent thereto and provided with an open end confronting said conical surface, said open end having an outwardly curled edge to minimize abrupt space potential gradients, the opening of said open end having a diameter approximately equal that of said conical surface and the spacing of said outwardly curved edge from the curved edge of said conical surface being about one half the maximum diameter of said conical surface and an apertured transverse wall partition coaxially positioned in said second cylindrical electrode and spaced from the said confronting open end thereof with its said aperture coaxial with said reference axis.

References Cited by the Examiner UNITED STATES PATENTS 2,275,029 3/1942 Epstein 315-l5 2,289,071 7/1942 Ramo 313-82 2,297,429 9/ 1942 Paehr 31382 2,825,837 3/1958 Dudley 3l382 3,134,919 5/1964 Schlesinger 3131-82 X GEORGE N. WESTBY, Primary Examiner.

P. C. DEMEO, Assistant Examiner.

cathode ray tube, a cathode I having an emitting surface arranged to emit electrons for 

1. IN A CATHODE RAY TUBE HAVING A REFERENCE ELECTRON KBEAM AXIS, A FIRST ELETRODE HAVING A CONCAVE CONICAL SURFACE OF ABOUT 109* APEX ANGLE COAXIAL WITH SAID REFERENCE AXIS SAID FIRST ELECTRODE HAVING AN APERTURE COAXIAL WITH SAID REFERENCE AXIS AT THE APEX OF SAID CONICAL SURFACE AND THROUGH WHICH AN ELECTRON BEAM CAN BE ADMITTED INTO THE REGION SURROUNDED BY SAID CONICAL SURFACE, THE PERIPHERAL PORTION OF SAID CONICAL SURFACE AT THE MAXIMUM DIAMETER THEREOF BEING OUTWARDLY AND REARWARDLY CURVED BACK UPON ITSELF TO FORM A ROUNDED PERIPHERAL EDGE ON SAID CONICAL SURFACE, AND AN ACCELERATING ELECTRODE HAVING A CYLINDRICAL PORTION CAOXIALLY SPACED FROM SAID CONICAL SURFACE AND HAVING AN OPEN END DISPOSED IN COAXIAL SPACED CONFRONTING RELATION WITH SAID CONICAL SURFACE, THE PERIPHERAL PORTION OF SAID OPEN END BEING OUTWARDLY AND REARWARDLY CURVED BACK UPON ITSELF TO FORM A ROUNDED PERIPHERAL EDGE ON SAID CYLINDRICAL PORTION, SAID OPEN END HAVING A DIAMETER APPROXIMATELY EQUAL THAT OF THE MAXIMUM DIAMETER OF SAID CONICAL SURFACE, THE AXIAL DISTANCE BETWEEN THE PERIPHERAL EDGE ON SAID CONICAL SURFACE AND THE PERIPHERAL EDGE ON SAID CYLINDRICAL PORTION BEING ABOUT 1/2 THE MAXIMUM DIAMETER OF SAID CONICAL SURFACE, THE MAXIMUM DIAMETER OF SAID CONICAL SURFACE BEING AT LEAST TEN TIMES THAT OF SAID APERTURE. 